• Energy Research

The L-mode density limit in ASDEX Upgrade is investigated with focus on the scrape-off layer. We observe a chain of events that involves the collapse of the E×B outer shear layer (just outside the separatrix), widening of the electron pressure fall-off length, outer divertor detachment, density shoulder formation, reduction and flattening of the parallel Mach number, and enhanced turbulent cross-field transport. The latter effect was associated with higher amplitude density fluctuations at low frequencies, where the turbulent cross-phase between density and velocity fluctuations pointing radially outward was favourable.

The electric field in the Scrape-Off Layer (SOL) of fusion plasmas is a key quantity affecting the local plasma transport and possibly also the overall plasma confinement. However, the physics determining the SOL electric field is experimentally not well investigated. In this study a systematic experimental analysis of the electric field in AUG L-mode discharges at various plasma currents, densities and heating powers is presented. In particular, the relation of the electric field to the divertor condition, as measured by Langmuir probes, is analyzed in detail by applying a simplified form of Ohm’s law. The analysis shows that the peak value of the radial electric field Er in the near SOL measured by Doppler reflectometry decreases from about 8kV/m at the lowest densities to -2kV/m at the highest densities, which is accompanied by a flattening of the outer divertor target electron temperature profiles. The electric field obtained by integrating Ohm’s law from the divertor target to the midplane agrees with Er in the near SOL within the uncertainties, evidencing the quantitative validity of the underlying model. Based on the findings about the relation between the electric field and the target conditions, a scaling to obtain the maximum of Er in the SOL in terms of upstream parameters is developed.

In fusion devices strong interest is deserved to the edge filament transport both related to turbulent and ELMy structures, because they are believed to provide important interaction with the plasma facing components and divertor plates. Among their features also the electromagnetic (EM) properties are expected to play an important role and in particular for the high beta regimes expected in future devices. The presence of ELM and inter-ELM electromagnetic filaments were detected during H-mode discharges in COMPASS device Scrape-Off Layer, where a new probe head was recently developed and commissioned. The diagnostic allows the simultaneous measurements of electrostatic and magnetic fluctuations with high time resolution, suitable for the identification of EM features of filaments. The method allows in particular the direct measurement of the current density associated to filaments. Detailed electrostatic and electromagnetic features analysis revealed a complex and fragmented structure within a single ELM and strong peaks in parallel current density Jtor are observed to characterize the ELM bunch. Analogous EM structures are observed also in inter-ELM phase, but with more than one order of magnitude lower intensity and time scale.